Superconducting thin film and a method for preparing the same

ABSTRACT

A high-temperature superconducting thin film of compound oxide selected from the group consisting of: 
     Y 1  Ba 2  Cu 3  O 7-x , Ho 1  Ba 2  Cu 3  O 7-x , Lu 1  Ba 2  Cu 3  O 7-x , 
     Sm 1  Ba 2  Cu 3  O 7-x , Nd 1  Ba 2  Cu 3  O 7-x , Gd 1  Ba 2  Cu 3  O 7-x , 
     Eu 1  Ba 2  Cu 3  O 7-x , Er 1  Ba 2  Cu 3  O 7-x , Dy 1  Ba 2  Cu 3  O 7-x , 
     Tm 1  Ba 2  Cu 3  O 7-x , Yb 1  Ba 2  Cu 3  O 7-x , La 1  Ba 2  Cu 3  O 7-x , 
     (La, Sr) 2  CuO 4-x , 
     which is deposited on a substrate of MgO or SrTiO 3 , with the outer surface of the high-temperature superconducting thin film being covered with a protective crystalline film of SrTiO 3 .

This application is a continuation of application Ser. No. 08/415,763filed Apr. 3, 1995 (now abandoned), which is a continuation ofapplication Ser. No. 07/215,497 filed Jul. 6, 1988 (now abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconducting thin film and amethod for preparing the same. More particularly, it relates to asuperconducting thin film of compound oxide having a high criticaltemperature and also possessing lasting stability for a long period anda method for preparing the same.

2. Description of the Related Art

The superconductivity is a phenomenon which is explained to be aphenomenon of a kind of phase change of electrons under which theelectrical resistance becomes zero and the perfect diamagnetism isobserved. Thus, under the superconducting condition, electric current ofa very high current density can be delivered without any loss of power.

Therefore, if the superconducting power cable is realized, the powerloss of the order of 7% which is inevitable in the conventional powercables can be reduced greatly. Realization of superconducting coils forgenerating a very high magnetic field is expected to acceleratedevelopment in the field of fusion power generation in which theelectric power is consumed beyond its output under the presenttechnology, as well as in the field of MHD power generation ormotor-generators. The development of superconductivity is demanded alsoin the other industrial fields such as in the field of electric powerreservation; in the field of transportation for example magneticlevitation trains, or magnetically propelling ships; in the medicalfield such as high-energy beam radiation unit; or in the field ofscience such as NMR or high-energy physics.

In addition to the abovementioned power electric applications, thesuperconducting materials can be used in the field of electronics, forexample, as a device using the Josephson effect in which quantumefficiency is observed macroscopically when an electric current ispassed through a weak junction arranged between two superconductingbodies. Tunnel junction type Josephson device which is a typicalapplication of the Josephson effect is expected to be a high-speed andlow-power consuming switching device owing to smaller energy gap of thesuperconducting material. It is also expected to utilize the Josephsondevice as a highly sensitive sensors or detectors for sensing very weakmagnetic field, microwave, radiant ray or the like since variation ofelectromagnetic wave or magnetic field is reflected in variation ofJosephson effect and can be observed as a quantum phenomenon precisely.Development of the superconducting devices is also demanded in the fieldof high-speed computers in which the power consumption per unit area isreaching to the upper limit of the cooling capacity with increment ofthe integration density in order to reduce energy consumption.

However, the critical temperature could not exceed 23.2K of Nb₃ Ge whichwas the the highest Tc for all studies for the past ten years.

Possibility of existence of a new type of superconducting materialshaving much higher Tc was revealed by Bednorz and Muller who discovereda new oxide type superconductor in 1986 Z. Phys. B64 (1986) 189!.

It had been known that certain ceramics material of compound oxidesexhibit the property of superconductivity. For example, U.S. Pat. No.3,932,315 discloses Ba--Pb--Bi type compound oxide which showssuperconductivity and Japanese patent laid-open No. 60-173,885 disclosesthat Ba--Bi type compound oxides also show superconductivity. Thesesuperconductors, however, possess rather lower transition temperaturesof about 10K and hence usage of liquidized helium (boiling point of4.2K) as cryogen is indispensable to realize superconductivity. The newtype compound oxide superconductor discovered by Bednorz and Muller isrepresented by La, Sr!₂ CuO₄ which is called as the K₂ NiF₄ -type oxidehaving a crystal structure which is similar to known perovskite typeoxides. The K₂ NiF₄ -type oxides show such higher Tc as 30K which areextremely higher than known superconducting materials.

It was also reported in the newspaper that C. W. Chu et al discovered inthe United States of America another superconducting material so calledYBCO type represented by YBa₂ Cu₃ O_(7-x) having the criticaltemperature of in the order of 90K in February 1987. Still another typenew superconducting material is a compound oxide of Bi--Sr--Ca--Cu--Osystem and Tl--Ba--Ca--Cu--O system which exhibit such high Tc as morethan 100K and which are chemically much more stable than theabovementioned YBCO type compound oxide or the like.

And hence, possibility of existence of high-temperature superconductorshave burst on the scene.

A thin film of superconducting compound oxide is prepared by sputteringtechnique. For example, in Japanese patent laid-open No. 56-109824 whichdiscloses a method for preparing a thin film of superconductorrepresented by BaPb_(1-x) Bi_(x) O₃ (in which 0.05≦x≦0.35),high-frequency sputtering is carried out in an oxygen containingatmosphere and then the resulting film is further heated at 500° to 550°C. We also described sputtering conditions for preparing superconductingthin films of compound oxide by the sputtering technique in a co-pendingU.S. patent Ser. No. 152,714 filed on May 2, 1988.

The abovementioned conventional sputtering technique itself issatisfactory but is helpless against the change in properties duringstorage.

In fact, the superconductivity or superconducting property of theabovementioned new type superconducting materials of compound oxide isinfluenced by the oxygen contents in the crystalline structure since theoxygen in the crystal is not stable and is apt to escape out of thecrystal, resulting in that the superconductivity is lost or deterioratedwith the passing of time.

Therefore, an object of the present invention is to overcome theabovementioned problems of the conventional technique and to provide asuperconducting thin film improved in stability during storage or useand a method for preparing the same.

SUMMARY OF THE INVENTION

A superconducting thin film composed of compound oxide and deposited ona substrate according to the present invention is characterized in thatan outer surface of the superconducting thin film is covered with aprotective layer which is composed of any one of the followingmaterials:

(i) oxide of metal,

(ii) carbide,

(iii) nitride or

(iv) the same material as the substrate.

The oxide of metal of (i) may be any oxide of an element selected from agroup comprising a metal element of Ha group, a metal element of IIIagroup, a metal element of IVa group, a metal element of Va group, ametal element of VIa group, a metal element of VIIa group, a metalelement of VIIIa group, a metal element of Ib group, a metal element ofIIb group, a semi-metal element of IIIb group and a semi-metal elementof IVb group in the periodic table. More particularly, it can bementioned one of oxides of following elements: Mg, Ca, Sr, Ba (IIagroup); Y, La, lanthanoid (IIIa group); Ti, Zr (IVa group); V, Nb, Ta(Va group); Cr, Mo, W (VIa group); Mn (VIIa group); Fe, Co, Ni (VIIIagroup); Cu, Ag (Ib group); Zn, Cd, Hg (IIb group); Al, Ga, In, Tl (IIIbgroup) and Si, Ge, Sn, Pb (IVb group).

According to the first aspect of the present invention, the protectivelayer of the present invention may be composed of oxide of at least oneof elements selected from a group comprising elements of Ha group, IIIagroup, IIIb group and IVb group, particularly, Mg, Ca, Sr, Ba, Zn, Cd,Hg, Al, Ga, In Tl, Si, Ge, Sn and Pb. Since the oxides of these elementsare homologue of or possess very similar properties to components of thesuperconductor, no undesirable influence is produced when they aredeposited on the surface of the superconductor. Still more, the oxidesof these elements contain stable oxygen which may advantageouslysuppress liberation of oxygen from the crystalline superconducting thinfilm, so that the superconducting property of the thin film can bemaintained.

According to second aspect of the present invention, the protectivelayer of the present invention may be composed of oxide of transitionmetal, particularly at least one metal selected from a group comprisingTi, Zr, V, Nb, Ta, Cr, Mo, W. Mn, Fe, Co, Ni, Cu and Ag. The oxides ofthese elements also contain stable oxygen which may advantageouslysuppress liberation of oxygen from the crystalline superconducting thinfilm to maintain the superconducting property of the thin film.

According to third aspect of the present invention, the protective layerof the present invention may be composed of oxide of such a metalelement or elements that are contained in the superconducting thin filmto be produced, such as Ba, Y, Sr or La. Each of these metal oxides is acomponent of the superconductor of compound oxide to be produced andhave no undesirable effect on the superconductor.

The carbide of (ii) may be SiC or TiC each of which may contain otherelements such as hydrogen. Since the carbide is chemically stable andhard and produce a protective layer having a high density which mayadvantageously suppress liberation of oxygen from the crystallinesuperconducting thin fin to maintain the superconducting property of thethin film.

The nitride of (iii) may be SiC or TiC each of which may contain otherelements such as hydrogen. Since the nitride is chemically stable orinactive to the superconductor and produce a protective layer having ahigh density which may advantageously suppress liberation of oxygen fromthe crystalline superconducting thin film, the superconducting propertyof the thin film can be maintained for a long time.

In case of (iv), the protective layer may be composed of the samematerial as the substrate which is made of a single crystal of MgO orSrTiO₃. When this material is deposited on the superconducting thinfilm, the deposited film grows epitaxially so that die crystalline axisof the resulting protective layer can be oriented to a predetermineddirection. This means that the resulting protective film has a very highdensity and can prevent liberation of oxygen out of the superconductingthin film. Of course, this protective layer is inactive to thesuperconductor.

Selection of the abovementioned protective layers depend on type orsystem of the compound oxide used.

The superconducting thin film can be composed of compound oxide of anelement a selected from IIa group of the Periodic Table, an element βselected from IIIa group of the Periodic Table and at least one elementγ selected from a group comprising Ib, IIb, IIIb, IVb and VIIIa group ofthe Periodic Table. The element γ is copper in general.

Particularly, the superconducting thin film is preferably a compoundoxide represented by the general formula:

    (α.sub.1-x β.sub.x)Cu.sub.y O.sub.z

in which α and β means the same definition as above, x is an atomicratio of β with respect to (α+β) and has a range of 0.1≦x≦0.9, and y andz are respective atomic ratios with respect to (α_(1-x) β_(x)) which isconsidered to be equal to 1 and each satisfy ranges of 0.4≦y≦3.0 and1≦z≦5 respectively.

According to a preferred embodiment of the present invention, theelement α is Ba or Sr and the element β is at least one element selectedfrom a group comprising Y, La, Gd, Dy, Ho, Er, Tm, Yb, Nd, Sm, Eu andLu. From 10 to 80% of the element α may be substituted by one or morethan one of elements selected from Ha group of the Periodic Table. Theelement β may be a combination of more than two elements selected fromIIIa group of the Periodic. A portion of these components can besubstituted by at least one of elements selected from a group comprisingAl, Fe, Ni, Zn, Ag and Ti.

An atomic ratio of α to β depends on the type or system of the compoundoxide or on the combination of α and β. Following atomic ratios arepreferably used in case of Ba--Y system, Ba--La system and Sr--La systemrespectively:

Y/(Y+Ba) 0.06 to 0.94, more preferably 0.1 to 0.4

Ba/(La+Ba) 0.04 to 0.96, more preferably 0.08 to 0.45

Sr/(La+Sr) 0.03 to 0.95, more preferably 0.05 to 0.1

Thus, the preferable compound oxides are Y--Ba--Cu--O system,La--Ba--Cu--O system and La--Sr--Cu--O system including the followingspecial cases:

Y₁ Ba₂ Cu₃ O_(7-x), Ho₁ Ba₂ Cu₃ O_(7-x), Lu₁ Ba₂ Cu₃ O_(7-x),

Sm₁ Ba₂ Cu₃ O_(7-x), Nd₁ Ba₂ Cu₃ O_(7-x), Gd₁ Ba₂ Cu₃ O_(7-x),

Eu₁ Ba₂ Cu₃ O_(7-x), Er₁ Ba₂ Cu₃ O_(7-x), Dy₁ Ba₂ Cu₃ O_(7-x),

Tm₁ Ba₂ Cu₃ O_(7-x), Yb₁ Ba₂ Cu₃ O_(7-x), La₁ Ba₂ Cu₃ O_(7-x),

(La, Sr)₂ CuO_(4-x)

in which x is a number which satisfies a range of 0<x<1.

The abovementioned oxides possess preferably perovskite type orquasi-perovskite type crystal structure. The term of quasi-perovskitetype means a structure which can be considered to have such a crystalstructure that is similar to Perovskite-type oxides and includes anorthorhombically distorted perovskite or a distorted oxygen-deficientperovskite or the like.

The superconducting thin film may be also another type of superconductorconsisting mainly of a compound oxide represented by the formula:

    Θ.sub.4 (Φ.sub.1-q, Ca.sub.q).sub.m Cu.sub.n O.sub.p+r

in which Θ stands for Bi or Tl, Θ stands for Sr when Φ is Bi and standsfor Ba when Φ is Tl, m and n are numbers each satisfying ranges of6≦m≦10 and 4≦n≦8 respectively, p=(6+m+n), q is a number which satisfiesa range of 0<q<1, and r is a number which satisfies a range of -2≦r≦+2.This system is considered to be a single phase of the following compoundoxide or a mixed phase consisting mainly thereof:

Bi₄ Sr₄ Ca₄ Cu₆ O_(20-r), Bi₂ Sr₂ Ca₂ Cu₃ O_(10-r),

Tl₄ Ba₄ Ca₄ Cu₆ O_(20-n), Tl₂ Ba₂ Ca₂ Cu₃ O_(10-r),

in which r is a number which satisfies a range of -2≦r≦+2.

The substrate may be made of glass, quartz, silicon, sapphire,stainless, steel or other ceramics. Particularly, the substrate consistspreferably of a single crystal of MgO or SrTiO₃. Desirably, thesuperconducting thin film is deposited on a (001) plane or (110) planeof a single crystal of MgO or SrTiO₃.

The present invention relates also to a method for preparing asuperconducting thin film on a substrate, The method is characterized inthat an outer surface of the superconducting thin film is covered with aprotective layer which is composed of any one of the followingmaterials:

(i) oxide of metal,

(ii) carbide,

(iii) nitride or

(iv) the same material as the substrate.

The thin film of superconductor can be prepared by the conventionalphysical vapour deposition technique, such as sputtering, vacuumdeposition, ion plating, molecular beam epitaxial growth or the like. Itis also possible to use chemical deposition technique (CVD) such asplasma CVD or photo CVD. Generally, the thin film of compound oxide isprepared by sputtering technique.

According to the present invention, more than two protective layers canbe formed successively on the superconducting thin film layer. Eachprotective layer may be different in type and composition from adjacentlayers.

According to a preferred embodiment of the present invention, thesuperconducting thin film is deposited on a substrate by sputteringtechnique and then a protective layer is deposited on a surface of thesuperconducting thin film in a common sputtering unit. Namely, thesputtering is preferably carried out by such a manner that, at first,both targets for the superconducting thin film and for the protectivelayer are set on respective target holders in a chamber, and then thetarget for the superconducting thin film is sputtered with argon gas orthe like while the target for the protective layer is not sputtered.After a superconducting thin film is deposited on a substrate, thetarget for the protective layer is sputtered to deposit a protectivelayer on the superconducting thin film.

Such superconducting thin film can be prepared by sputtering techniquedescribed in our co-pending U.S. patent Ser. No. 152,714 filed on May 2,1988.

The superconducting property can be improved by heat-treatment which iseffected after the thin film of compound oxide is deposited on thesubstrate in oxygen containing atmosphere. The heat-treatment ispreferably effected under a partial pressure of oxygen ranging from 0.1to 150 atm at a temperature between 300° and 1,500° C. After thistemperature is maintained for more than one hour, the resulting thinfilm is cooled slowly at a rate of less than 100° C./min. Advantage ofthe heat-treatment can not be obtained if the heat-treatment is effectedoutside the abovementioned conditions. For example, if the in film isheated at a temperature which is higher than 1,500° C., theabovementioned advantage can not be obtained but the superconductivitywill be disappeared.

In case of the compound oxide type superconductor, oxygen deficiency inits crystal is a critical factor for realizing the superconductivity.Particularly, a heat-treatment under a relatively higher partialpressure of oxygen is very preferable and is considered to beindispensable to realize superior superconductivity.

However, it is impossible to prevent the thin film of compound oxidefrom deterioration which occur during storage even if the thin film isheat-treated completely. The deterioration of superconductivity, inother words a phenomenon that the superconducting property is lostgradually in time may be caused by disappearance or liberation of oxygenin the crystal because the superconductivity of compound oxide isobserved in unstable or quasi-stable phase. This is a big problem ofcompound oxide type superconductors in their actual use.

This problem is solved by the present invention in which an outersurface of the thin film of superconductor is covered by a stable andhard protective layer which has a high density to prevent oxygen fromescaping from its crystal structure.

In a stage of deposition of the protective layer on the superconductingthin film, the substrate on which the thin film is deposited should notbe heated at a temperature which is higher than 500° C. In fact, if thesubstrate is heated at a temperature which is higher than 500° C., thesuperconducting property is damaged because oxygen in its crystal is aptto be lost.

The thin film of compound oxide is preferably deposited on a (001) planeor (110) plane of a single crystal of MgO or SrTiO₃. In fact, the thinfilm of compound oxide possesses anisotropy in its crystal structure, sothat a current passing along the direction which is in parallel with aplane defined by a-axis and b-axis of the crystal show a very highcritical current density but the other current passing along the otherdirections is relatively lower. Therefore, when the c-axis is orientatedto a direction which is perpendicular to the plane defined by bra-axisand b-axis of the crystal, a current which has much higher criticalcurrent density (Jc) can be delivered along a direction which is inparallel wit the plane.

According to a preferred embodiment of the present invention, thesuperconducting thin film composed of a single crystal or polycrystalsof compound oxide whose c-axis is oriented to a direction which isperpendicular to the plane defined by by a-axis and b-axis of thecrystal is deposited on a (100) plane of a substrate of single crystalof MgO or SrTiO₃ which has such lattice constant or lattice constants ina-axis and/or b-axis that are proximate to those of the in film such asLn₁ Ba₂ Cu₃ O₇ in order to realize a high critical current density (Jc)along the direction which Is in parallel with a surface of thesubstrate.

In case of (iv) where the protective layer is composed of the samematerial as the substrate which consist of a single crystal of MgO orSrTiO₃, the deposited protective layer grows epitaxially so that thecrystalline axis of the resulting protective layer is oriented to apredetermined direction, resulting in that protective film has a veryhigh density and can prevent escape of oxygen out of the superconductingthin film.

It is apparent from the description abovementioned that thesuperconducting thin film of compound oxide according to the presentinvention have improved stability than conventional superconducting thinfilm and hence they can be utilized advantageously in a applications ofthin film devices, such as Matisoo switching elements or Josephsondevice, Anacker memory device or Superconducting Quantum InterferenceDevice (SQUID).

Now, the present invention will be described with reference to attacheddrawings which illustrate an apparatus used for carrying out the presentinvention. But, the present invention is not limited thereto.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an illustrative view of a magnetron sputtering unit which isused in the present invention.

Referring to FIG. 1 illustrating a magnetron sputtering unit which isused in the present invention, the sputtering unit includes, in achamber 1, two target holders for a superconducting thin film 2 and fora protective layer 3, a pair of magnetron electrodes 4 each surroundingrespective targets 2 and 3, high-frequency power sources 5 for actuatingthe magnetron electrodes 5 and a substrate holder on which a substrateis secured. The chamber 1 has an exhaust port 8 which is connected to avacuum source (not shown) and a gas inlet port 7 for introducing anatmosphere gas. The substrate holder is provided with a heater 9 forcontrolling a temperature of the substrate 6. Such sputtering unititself are known and used in a variety of applications.

In operation, after a substrate 6 is secured to the substrate holder andtargets 2 and 3 are set on respective target holder in the chamber 1,vacuum is created in the chamber 1 and a suitable gas such as argonand/or oxygen at a predetermined pressure is introduced into thechamber 1. After the substrate 6 is heated to a predeterminedtemperature by the heater 9, high-frequency power is applied to one ofthe magnetron electrode 4 surrounding the target 2 for thesuperconducting thin film to start sputtering operation. Introduction ofthe atmosphere gas in the chamber 1 is controlled at a predeterminedpressure during the sputtering operation. The deposition of thesuperconducting thin film of compound oxide is carried out by theconventional sputtering technique.

After the thin film of compound oxide is deposited, another themagnetron electrode 4 surrounding the target 3 for the protective layeris energized so that a thin film of the protective layer is deposited onthe super conducting thin film.

Now, several embodiments of the method according to the presentinvention will be described by Examples, but the scope of the presentinvention should not be limited thereto.

In the following Examples, both of the superconducting thin film and theprotective layer are prepared in the sputtering unit shown in FIG. 1.For comparison, two series of samples are prepared so that the firstseries possesses the protective layer while second series do not possessthe same.

EXAMPLE 1

Preparation of a Target for Compound Oxide

At first, powders of Y₂ O₃ and BaCO₃ were mixed at an atomic ratio ofY:Ba=1:2 and then a powder of CuO was added to the resulting powdermixture at 10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu 1:2:3. Then, the resulting powder mixture was sintered at 950°C. to obtain a sintered block of YBa₂ Cu₃ O₇ which is used as a target 2for a superconducting in film,

Deposition of Thin Films

The resulting target 2 and a target 3 composed of h-BN are set onrespective target holder and a substrate 6 consisting of a singlecrystal of MgO is secured on the substrate holder. The MgO crystal isset in such manner that its surface on which the thin film is depositedhas a (001) plane.

After the chamber 1 is vacuumed, argon gas at a partial pressure of5.0×10⁻² Torr and oxygen gas at a partial pressure of 1.0×10⁻² Torr areintroduced and the substrate is heated at 620° C. Then, the magnetronelectrode 4 for the target 2 is energized with high frequency of 3 W/cm²to prepare a thin film of compound oxide of 1 μm on the substrate at afilm forming rate of 0.50 Å/sec.

After deposition of the thin film of compound oxide complete, oxygen gasof 1 atom is introduced in the chamber 1 and the temperature of thesubstrate is adjusted at 650° C. The deposited thin film is left underthis condition for 15 hour and then cooled slowly at a cooling rate of7° C./min.

Then, another magnetron electrode for the protective layer 3 isenergized so that h-BN is sputtered to deposit a protective layer of2,000 Å on the thin film of compound oxide which is prepared in theabovementioned step. During the deposition of the protective layer, thetemperature of the substrate is maintained at 450° C.

Measurement of Critical Temperature

Resistance of the resulting thin film is measured on such a sample thathas vacuum-deposited aluminum electrodes at opposite sides of the thinfilm of compound oxide deposited on the substrate 6.

Measurement of the critical temperature Tc and Tcf was carried out by aconventional four probe method in which the sample was immersed inliquidized helium to cool the sample down to a temperature of 8K in acryostat. Then, the temperature dependence of resistance of the samplewas determined with rising the temperature gradually to determine atemperature of Tcf at which the perfect superconductivity start to belost and resistance begin to appear and a temperature of Tc at which thesuperconductivity is lost and ordinary resistance begin to appear.

Changes of Tcf and Tc are determined by comparing two values observed onsame sample just after the protective layer is deposited and after onemonth.

The result as well as main operational parameters are shown in Table 1.

EXAMPLE 2

The same procedure as Example 1 is repeated except that a sintered blockof LaBa₂ Cu₃ O₇ is used as a target for a thin film of compound oxidewhich is prepared by following procedure.

Preparation of a Target for Compound Oxide

Powders of La₂ O₃ and BaCO₃ were mixed at an atomic ratio of La:Ba=1:2and then a powder of CuO was added to the resulting powder mixture at10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970°C. to obtain a sintered block of LaBa₂ Cu₃ O₇ which is used as a target2 for a superconducting thin film.

The result as well as main operational parameters are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                              One                                     Partial pressure                                                                          Temperature        Just after                                                                           month                                   of          of                 deposition                                                                           later                                        O.sub.2    Substrate Protective                                                                           Tc   Tcf Tc   Tcf                            No.  (Torr)     (°C.)                                                                            Layer  (K)  (K) (K)  (K)                            ______________________________________                                        1    1.0 × 10.sup.-2                                                                    620       BN     91   78  90   78                                                       --     89   77  36   --                             2    7.0 × 10.sup.-3                                                                    680       BN     58   47  59   48                                                       --     57   48   8   --                             ______________________________________                                         (Note)--: Superconductivity is not observed in liquid helium             

EXAMPLE 3

The same procedure as Example 1 is repeated except that the target ofh-BN is replaced by a target of TiN.

The result as well as main operational parameters are shown in Table 2.

EXAMPLE 4

The same procedure as Example 2 is repeated except that the target ofh-BN is replaced by a target of TiN.

The result as well as main operational parameters are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                              One                                     Partial pressure                                                                          Temperature        Just after                                                                           month                                   of          of                 deposition                                                                           later                                        O.sub.2    Substrate Protective                                                                           Tc   Tcf Tc   Tcf                            No.  (Torr)     (°C.)                                                                            Layer  (K)  (K) (K)  (K)                            ______________________________________                                        3    8.0 × 10.sup.-3                                                                    730       TiN    96   85  95   86                                                       --     98   88  33   --                             4    5.0 × 10.sup.-3                                                                    690       TiN    51   38  53   37                                                       --     49   37  17   --                             ______________________________________                                         (Note)--: Superconductivity is not observed in liquid helium             

EXAMPLE 5

Preparation of a Target for Compound Oxide

At first, powders of Y₂ O₃ and BaCO₃ were mixed at an atomic ratio ofY:Ba=1:2 and then a powder of CuO was added to the resulting powdermixture at 10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950°C. to obtain a sintered block of YBa₂ Cu₃ O₇ which is used as a target 2for a superconducting thin film.

Deposition of Thin Films

The resulting target 2 and a target 3 composed of SiC are set onrespective target holder and a substrate 6 consisting of a singlecrystal of MgO is secured on the substrate holder. The MgO crystal isset in such manner that its surface on which the thin film is depositedhas a (001) plane.

After the chamber 1 is vacuumed, argon gas at a partial pressure of5.0×10⁻² Torr and oxygen gas at a partial pressure of 1.0×10⁻² Torr areintroduced and the substrate is heated at 630° C. Then, the magnetronelectrode 4 for the target 2 is energized with high frequency of 3 W/cm²to prepare a thin film of compound oxide of 1 μm on the substrate at afilm forming rate of 0.50 Å/sec.

After deposition of the thin film of compound oxide complete, oxygen gasof 1 atom is introduced in the chamber 1 and the temperature of thesubstrate is adjusted at 650° C. The deposited thin film is left underthis condition for 15 hour and then cooled slowly at a cooling rate of7° C./min.

Then, another magnetron electrode for the protective layer 3 isenergized so that SiC is sputtered to deposit a protective layer of2,000 Å on the thin film of compound oxide which is prepared in theabovementioned step. During the deposition of the protective layer, thetemperature of the substrate is maintained at 450° C.

Measurement of Critical Temperature

Measurement of the critical temperature Tc and Tcf was carried by thesame method as Example 1. The result as well as main operationalparameters are shown in Table 3.

EXAMPLE 6

The same procedure as Example 5 is repeated except that a sintered blockof LaBa₂ Cu₃ O₇ is used as a target for a thin film of compound oxidewhich is prepared by following procedure.

Preparation of a Target for Compound Oxide

Powders of La₂ O₃ and BaCO₃ were mixed at an atomic ratio of La:Ba=1:2and then a powder of CuO was added to the resulting powder mixture at10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970°C. to obtain a sintered block of LaBa₂ Cu₃ O₇ which is used as a target2 for a superconducting thin film.

The result as well as main operational parameters are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                                              One                                     Partial pressure                                                                          Temperature        Just after                                                                           month                                   of          of                 deposition                                                                           later                                        O.sub.2    Substrate Protective                                                                           Tc   Tcf Tc   Tcf                            No.  (Torr)     (°C.)                                                                            Layer  (K)  (K) (K)  (K)                            ______________________________________                                        5    1.0 × 10.sup.-2                                                                    630       SiC    86   72  87   71                                                       --     85   72  33   --                             6    7.0 × 10.sup.-3                                                                    670       SiC    61   47  60   48                                                       --     62   50  --   --                             ______________________________________                                         (Note)--: Superconductivity is not observed in liquid helium             

EXAMPLE 7

The same procedure as Example 5 is repeated except that the target ofSiC is replaced by a target of TiC.

The result as well as main operational parameters are shown in Table 4.

EXAMPLE 8

The same procedure as Example 6 is repeated except that the target ofSiC is replaced by a target of TiC.

The result as well as main operational parameters are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                                              One                                     Partial pressure                                                                          Temperature        Just after                                                                           month                                   of          of                 deposition                                                                           later                                        O.sub.2    Substrate Protective                                                                           Tc   Tcf Tc   Tcf                            No.  (Torr)     (°C.)                                                                            Layer  (K)  (K) (K)  (K)                            ______________________________________                                        7    8.0 × 10.sup.-3                                                                    710       TiC    93   82  94   81                                                       --     91   80  31   --                             8    5.0 × 10.sup.-3                                                                    685       TiC    53   39  51   38                                                       --     52   38  --   --                             ______________________________________                                         (Note)--: Superconductivity is not observed in liquid helium             

EXAMPLE 9

Preparation of a Target for Compound Oxide

At first, powders of Y₂ O₃ and BaCO₃ were mixed at an atomic ratio ofY:Ba=1:2 and then a powder of CuO was added to the resulting powdermixture at 10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950°C. to obtain a sintered block of YBa₂ Cu₃ O₇ which is used as a target 2for a superconducting thin film,

Deposition of Thin Films

The resulting target 2 and a powder target 3 composed of MgO powder areset on respective target holder and a substrate 6 consisting of a singlecrystal of MgO is secured on the substrate holder. The substrate of MgOcrystal is set in such manner that its surface on which the thin film isdeposited has a (001) plane.

After the chamber 1 is vacuumed, argon gas at a partial pressure of5.0×10⁻² Torr and oxygen gas at a partial pressure of 1.0×10⁻² Torr areintroduced and the substrate is heated at 650° C. Then, the magnetronelectrode 4 for the target 2 is energized with high frequency of 3 W/cm²to prepare a thin film of compound oxide of 1 μm on the substrate at afilm forming rate of 0.50 Å /sec.

After deposition of the thin film of compound oxide complete, oxygen gasof 1 atom is introduced in the chamber 1 and the temperature of thesubstrate is adjusted at 650° C. The deposited thin film is left underthis condition for 15 hour and then cooled slowly at a cooling rate of7° C./min.

Then, another magnetron electrode for the protective layer 3 isenergized so that MgO is sputtered to deposit a protective layer of2,000 Å on the thin film of compound oxide which is prepared in theabovementioned step. During the deposition of the protective layer, thetemperature of the substrate is maintained at 450° C.

Measurement of Critical Temperature

Measurement of the critical temperature Tc and Tcf was carried by thesame method as Example 1. The result as welt as main operationalparameters are shown in Table 5.

EXAMPLE 10

The same procedure as Example 9 is repeated except that a sintered blockof LaBa₂ Cu₃ O₇ is used as a target for a thin film of compound oxidewhich is prepared by following procedure.

Preparation of a Target for Compound Oxide

Powders of La₂ O₃ and BaCO₃ were mixed at an atomic ratio of La:Ba=1:2and then a powder of CuO was added to the resulting powder mixture at10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970°C. to obtain a sintered block of LaBa₂ Cu₃ O₇ which is used as a target2 for a superconducting thin film.

The result as well as main operational parameters are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                                           Just                                                                          after After                                Partial    Temperature             Deposi-                                                                             1                                    pressure   of        Protec-       tion  month                                     of O.sub.2                                                                              Substrate tive  Jc    Tc  Tcf Tc  Tcf                          No.  (Torr)    (°C.)                                                                            Layer (A/cm.sup.2)                                                                        (K) (K) (K) (K)                          ______________________________________                                         9   1.0 × 10.sup.-2                                                                   650       MgO   3 × 10.sup.6                                                                  79  67  77  68                                                    --    1 × 10.sup.5                                                                  78  68  11  --                           10   7.0 × 10.sup.-3                                                                   620       MgO   5 × 10.sup.5                                                                  56  39  56  38                                                    --    8 × 10.sup.3                                                                  55  40  --  --                           ______________________________________                                         (Note)--: Superconductivity is not observed in liquid helium             

EXAMPLE 11

The same procedure as Example 9 is repeated except that the powdertarget of MgO is replaced by a target of SrTiO₃ block.

The result as well as main operational parameters are shown in Table 6.

EXAMPLE 12

The same procedure as Example 10 is repeated except that the substrateof MgO is replaced by a substrate composed of a single crystal of SrTiO₃and that the target of powder MgO is also replaced by a target ofSrTiO₃. The thin film of compound oxide is deposited on (100) plane ofthe substrate of SrTiO₃ block.

The result as well as main operational parameters are shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                                           Just                                                                          after After                                Partial    Temperature             Deposi-                                                                             1                                    pressure   of        Protec-       tion  month                                     of O.sub.2                                                                              Substrate tive  Jc    Tc  Tcf Tc  Tcf                          No.  (Torr)    (°C.)                                                                            Layer (A/cm.sup.2)                                                                        (K) (K) (K) (K)                          ______________________________________                                        11   8.0 × 10.sup.-3                                                                   700       SrTiO.sub.3                                                                         2 × 10.sup.6                                                                  89  76  88  77                                                    --    9 × 10.sup.4                                                                  90  78  25  --                           12   5.0 × 10.sup.-3                                                                   640       SrTiO.sub.3                                                                         7 × 10.sup.5                                                                  49  34  48  35                                                    --    3 × 10.sup.4                                                                  48  26  --  --                           ______________________________________                                         (Note)--: Superconductivity is not observed in liquid helium             

EXAMPLE 13

Preparation of a Target for Compound Oxide

At first, powders of Y₂ O₃ and BaCO₃ were mixed at an atomic ratio ofY:Ba=1:2 and then a powder of CuO was added to the resulting powdermixture at 10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950°C. to obtain a sintered block of YBa₂ Cu₃ O₇ which is used as a target 2for a superconducting thin film.

Deposition of Thin Films

The resulting target 2 and a target 3 composed of MgO are set onrespective target holder and a substrate 6 consisting of a singlecrystal of MgO is secured on the substrate holder. The MgO crystal isset in such manner that its surface on which the thin film is depositedhas a (001) plane.

After the chamber 1 is vacuumed, argon gas at a partial pressure of5.0×10⁻² Torr and oxygen gas at a partial pressure of 1.0×10² Torr areintroduced and the substrate is heated at 600° C. Then, the magnetronelectrode 4 for the target 2 is energized with high frequency of 3 W/cm²to prepare a thin film of compound oxide of 1 μm on the substrate at afilm forming rate of 0.50 Å/sec.

After deposition of the thin film of compound oxide complete, oxygen gasof 1 atom is introduced in the chamber 1 and the temperature of thesubstrate is adjusted at 650° C. The deposited thin film is left underthis condition for 15 hour and then cooled slowly at a cooling rate of7° C./min.

Then, another magnetron electrode for the protective layer 3 isenergized so that MgO is sputtered to deposit a protective layer of2,000 Å on the thin film of compound oxide which is prepared in theabovementioned step. During the deposition of the protective layer, thetemperature of the substrate is maintained at 450° C.

Measurement of Critical Temperature

Measurement of the critical temperature Tc and Tcf was carried by thesame method as Example 1. The result as well as main operationalparameters are shown in Table 7.

EXAMPLE 14

The same procedure as Example 13 is repeated except that a sinteredblock of LaBa₂ Cu₃ O₇ is used as a target for a thin film of compoundoxide which is prepared by following procedure.

Preparation of a Target for Compound Oxide

Powders of La₂ O₃ and BaCO₃ were mixed at an atomic ratio of La:Ba=1:2and then a powder of CuO was added to the resulting powder mixture at10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970°C. to obtain a sintered block of LaBa₂ Cu₃ O₇ which is used as a target2 for a superconducting thin film.

The result as well as main operational parameters are shown in Table 7.

                  TABLE 7                                                         ______________________________________                                                                               One                                    Partial pressure                                                                           Temperature        Just after                                                                           month                                  of           of                 deposition                                                                           later                                       O.sub.2     Substrate Protective                                                                           Tc   Tcf Tc  Tcf                            No.  (Torr)      (°C.)                                                                            Layer  (K)  (K) (K) (K)                            ______________________________________                                        13   1.0 × 10.sup.-2                                                                     600       MgO    89   71  90  72                                                        --     77   68  39  --                             14   7.0 × 10.sup.-3                                                                     700       MgO    54   36  55  37                                                        --     55   34  18  --                             ______________________________________                                         (Note) -: Superconductivity is not observed in liquid helium             

EXAMPLE 15

The same procedure as Example 13 is repeated except that the target ofMgO is replaced by a target of SiO₂.

The result as well as main operational parameters are shown in Table 8.

EXAMPLE 16

The same procedure as Example 14 is repeated except that the target ofMgO is replaced by a target of SiO₂.

The result as well as main operational parameters are shown in Table 8.

                  TABLE 8                                                         ______________________________________                                                                               One                                    Partial pressure                                                                           Temperature        Just after                                                                           month                                  of           of                 deposition                                                                           later                                       O.sub.2     Substrate Protective                                                                           Tc   Tcf Tc  Tcf                            No.  (Torr)      (°C.)                                                                            Layer  (K)  (K) (K) (K)                            ______________________________________                                        15   8.0 × 10.sup.-3                                                                     650       SiO.sub.2                                                                            95   77  94  78                                                        --     92   75  34  --                             16   5.0 × 10.sup.-3                                                                     680       SiO.sub.2                                                                            47   35  48  37                                                        --     46   36  20  --                             ______________________________________                                         (Note) -: Superconductivity is not observed in liquid helium             

EXAMPLE 17

Preparation of a Target for Compound Oxide

At first, powders of Y₂ O₃ and BaCO₃ were mixed at an atomic ratio ofY:Ba=1:2 and then a powder of CuO was added to the resulting powdermixture at 10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950°C. to obtain a sintered block of YBa₂ Cu₃ O₇ which is used as a target 2for a superconducting thin film.

Deposition of Thin Films

The resulting target 2 and a target 3 composed of TiO₂ are set onrespective target holder and a substrate 6 consisting of a singlecrystal of MgO is secured on the substrate holder. The MgO crystal isset in such manner that its surface on which the thin film is depositedhas a (001) plane.

After the chamber 1 is vacuumed, argon gas at a partial pressure of5.0×10⁻² Torr and oxygen gas at a partial pressure of 1.0×10⁻² Torr areintroduced and the substrate is heated at 600° C. Then, the magnetronelectrode 4 for the target 2 is energized with high frequency of 3 W/cm²to prepare a thin film of compound oxide of 1 μm on the substrate at afilm forming rate of 0.50 Å/sec.

After deposition of the thin film of compound oxide complete, oxygen gasof 1 atom is introduced in the chamber 1 and the temperature of thesubstrate is adjusted at 650° C. The deposited thin film is left underthis condition for 15 hour and then cooled slowly at a cooling rate of7° C./min.

Then, another magnetron electrode for the protective layer 3 isenergized so that TiO₂ is sputtered to deposit a protective layer of2,000 Å on the thin film of compound oxide which is prepared in theabovementioned step. During the deposition of the protective layer, thetemperature of the substrate is maintained at 450° C.

Measurement of Critical Temperature

Measurement of the critical temperature Tc and Tcf was carried out bythe same method as Example 1. The result as well as main operationalparameters are shown in Table 9.

EXAMPLE 18

The same procedure as Example 17 is repeated except that a sinteredblock of LaBa₂ Cu₃ O₇ is used as a target for a thin film of compoundoxide which is prepared by following procedure.

Preparation of a Target for Compound Oxide

Powders of La₂ O₃ and BaCO₃ were mixed at an atomic ratio of La:Ba=1:2and then a powder of CuO was added to the resulting powder mixture at10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970°C. to obtain a sintered block of LaBa₂ Cu₃ O₇ which is used as a target2 for a superconducting thin foam.

The result as well as main operational parameters are shown in Table 9.

                  TABLE 9                                                         ______________________________________                                                                               One                                    Partial pressure                                                                           Temperature        Just after                                                                           month                                  of           of                 deposition                                                                           later                                       O.sub.2     Substrate Protective                                                                           Tc   Tcf Tc  Tcf                            No.  (Torr)      (°C.)                                                                            Layer  (K)  (K) (K) (K)                            ______________________________________                                        17   1.0 × 10.sup.-2                                                                     600       TiO.sub.2                                                                            88   72  89  71                                                        --     79   67  41  --                             18   7.0 × 10.sup.-3                                                                     700       TiO.sub.2                                                                            57   39  56  41                                                        --     58   36  21  --                             ______________________________________                                         (Note) -: Superconductivity is not observed in liquid helium             

EXAMPLE 19

The same procedure as Example 17 is repeated except that the target ofTiO₂ is replaced by a target of ZrO₂.

The result as well as main operational parameters are shown in Table 10.

EXAMPLE 20

The same procedure as Example 18 is repeated except that the target ofTiO₂ is replaced by a target of ZrO₂.

The result as well as main operational parameters are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                                                               One                                    Partial pressure                                                                           Temperature        Just after                                                                           month                                  of           of                 deposition                                                                           later                                       O.sub.2     Substrate Protective                                                                           Tc   Tcf Tc  Tcf                            No.  (Torr)      (°C.)                                                                            Layer  (K)  (K) (K) (K)                            ______________________________________                                        19   8.0 × 10.sup.-3                                                                     650       ZrO.sub.2                                                                            93   77  95  76                                                        --     91   76  38  --                             20   5.0 × 10.sup.-3                                                                     680       ZrO.sub.2                                                                            48   36  49  37                                                        --     47   34  21  --                             ______________________________________                                         (Note) -: Superconductivity is not observed in liquid helium             

EXAMPLE 21

Preparation of a Target for Compound Oxide

At first, powders of Y₂ O₃ and BaCO₃ were mixed at an atomic ratio ofY:Ba=1:2 and then a powder of CuO was added to the resulting powdermixture at 10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 950°C. to obtain a sintered block of YBa₂ Cu₃ O₇ which is used as a target 2for a superconducting thin film.

Deposition of Thin Films

The resulting target 2 and a target 3 composed of Y₂ O₃ are set onrespective target holder and a substrate 6 consisting of a singlecrystal of MgO is secured on the substrate holder. The MgO crystal isset in such manner that its surface on which the thin film is depositedhas a (001) plane.

After the chamber 1 is vacuumed, argon gas at a partial pressure of5.0×10⁻² Torr and oxygen gas at a partial pressure of 1.0×10⁻² Torr areintroduced and the substrate is heated at 680° C. Then, the magnetronelectrode 4 for the target 2 is energized with high frequency of 3 W/cm²to prepare a thin film of compound oxide of 1 μm on the substrate at afilm forming rate of 0.50 Å/sec.

After deposition of the thin film of compound oxide complete, oxygen gasof 1 atom is introduced in the chamber 1 and the temperature of thesubstrate is adjusted at 650° C. The deposited thin film is left underthis condition for 15 hour and then cooled slowly at a cooling rate of7° C./min.

Then, another magnetron electrode for the protective layer 3 isenergized so that Y₂ O₃ is sputtered to deposit a protective layer of2,000 Å on the thin film of compound oxide which is prepared in theabovementioned step. During the deposition of the protective layer, thetemperature of the substrate is maintained at 430° C.

Measurement of Critical Temperature

Measurement of the critical temperature Tc and Tcf was carried out bythe same method as Example 1. The result as well as main operationalparameters are shown in Table 11.

EXAMPLE 22

The same procedure as Example 21 is repeated except that a sinteredblock of LaBa₂ Cu₃ O₇ is used as a target for a thin film of compoundoxide which is prepared by following procedure.

Preparation of a Target for Compound Oxide

Powders of La₂ O₃ and BaCO₃ were mixed at an atomic ratio of La:Ba=1:2and then a powder of CuO was added to the resulting powder mixture at10% excessive proportion with respect to an atomic ratio ofY:Ba:Cu=1:2:3. Then, the resulting powder mixture was sintered at 970°C. to obtain a sintered block of LaBa₂ Cu₃ O₇ which is used as a target2 for a superconducting thin film.

Deposition of Thin Films

The resulting target 2 and a target 3 composed of La₂ O₃ are set onrespective target holder and a substrate 6 consisting of a singlecrystal of MgO is secured on the substrate holder. The MgO crystal isset in such manner that its surface on which the thin film is depositedhas a (001) plane.

After the chamber 1 is vacuumed, argon gas at a partial pressure of5.0×10⁻² Torr and oxygen gas at a partial pressure of 1.0×10⁻² Torr areintroduced and the substrate is heated at 680° C. Then, the magnetronelectrode 4 for the target 2 is energized with high frequency of 3 W/cm²to prepare a thin film of compound oxide of 1 μm on the substrate at afilm forming rate of 0.50 Å/sec.

After deposition of the thin film of compound oxide complete, oxygen gasof 1 atom is introduced in the chamber 1 and the temperature of thesubstrate is adjusted at 730° C. The deposited thin film is left underthis condition for 20 hour and then cooled slowly at a cooling rate of8° C./min.

Then, another magnetron electrode for the protective layer 3 isenergized so that La₂ O₃ is sputtered to deposit a protective layer of2,000 Å on the thin film of compound oxide which is prepared in theabovementioned step. During the deposition of the protective layer, thetemperature of the substrate is maintained at 380° C.

The result as well as main operational parameters are shown in Table 11.

                  TABLE 11                                                        ______________________________________                                                                               One                                    Partial pressure                                                                           Temperature        Just after                                                                           month                                  of           of                 deposition                                                                           later                                       O.sub.2     Substrate Protective                                                                           Tc   Tcf Tc  Tcf                            No.  (Torr)      (°C.)                                                                            Layer  (K)  (K) (K) (K)                            ______________________________________                                        21   1.0 × 10.sup.-2                                                                     680       Y.sub.2 O.sub.3                                                                      88   72  89  71                                                        --     79   67  41  --                             22   7.0 × 10.sup.-3                                                                     710       La.sub.2 O.sub.3                                                                     57   39  56  41                                                        --     58   36  11  --                             ______________________________________                                         (Note) -: Superconductivity is not observed in liquid helium             

EXAMPLE 23

The same procedure as Example 21 is repeated except that the target ofY₂ O₃ is replaced by a target of BaTiO₃.

The result as well as main operational parameters are shown in Table 12.

EXAMPLE 24

The same procedure as Example 22 is repeated except that the target ofY₂ O₃ is replaced by a target of BaTiO₃.

The result as well as main operational parameters are shown in Table 12.

                  TABLE 12                                                        ______________________________________                                                                               One                                    Partial pressure                                                                           Temperature        Just after                                                                           month                                  of           of                 deposition                                                                           later                                       O.sub.2     Substrate Protective                                                                           Tc   Tcf Tc  Tcf                            No.  (Torr)      (°C.)                                                                            Layer  (K)  (K) (K) (K)                            ______________________________________                                        23   8.0 × 10.sup.-3                                                                     640       BaTiO.sub.3                                                                          93   77  95  76                                                        --     91   76  38  --                             24   5.0 × 10.sup.-3                                                                     670       BaTiO.sub.3                                                                          48   36  49  37                                                        --     47   34  --  --                             ______________________________________                                         (Note) -: Superconductivity is not observed in liquid helium             

What is claimed are:
 1. A high-temperature superconducting thin filmcomposed of compound oxide and deposited on a substrate of MgO orSrTiO₃, wherein said compound oxide is a member selected from the groupconsisting of:Y₁ Ba₂ Cu₃ O_(7-x), Ho₁ Ba₂ Cu₃ O_(7-x), Lu₁ Ba₂ Cu₃O_(7-x), Sm₁ Ba₂ Cu₃ O_(7-x), Nd₁ Ba₂ Cu₃ O_(7-x), Gd₁ Ba₂ Cu₃ O_(7-x),Eu₁ Ba₂ Cu₃ O_(7-x), Er₁ Ba₂ Cu₃ O_(7-x), Dy₁ Ba₂ Cu₃ O_(7-x), Tm₁ Ba₂Cu₃ O_(7-x), Yb₁ Ba₂ Cu₃ O_(7-x), La₁ Ba₂ Cu₃ O_(7-x), (La, Sr)₂CuO_(4-x),in which x is a number which satisfies a range of 0<x<1, andwherein an outer surface of said high-temperature superconducting thinfilm is covered with a protective layer comprising a crystalline filmwhich is composed of SrTiO₃.
 2. A superconducting thin film set forth inclaim 1, wherein said superconducting thin film has a crystal structureof perovskite or quasi-perovskite.
 3. A superconducting thin film setforth in claim 1, wherein said substrate is a single crystal of MgO orSrTiO₃.
 4. A superconducting thin film set forth in claim 3, whereinthat said superconducting thin film is deposited on a (001) plane or(110) plane of a single crystal of MgO or SrTiO₃.